This invention relates to a method for mounting printing plates of substantially transparent polymer on the plate cylinders of flexographic printing machines.
As is well known to one skilled in the art, when a flexographic printing machine is to be used to print writing, markings or designs on a polymer strip which are to be repeated along the strip, various operations are required, and in particular:
the polymer printing plates must be prepared and their completeness checked by comparison with the sample or sketch;
the various plates must be mounted on the relative flexographic machine cylinders (one per colour), this mounting being achieved by double-adhesive tape of a width sufficient to cover a whole plate or a group of plates;
the plate cylinders must be mounted on the relative flexographic printing machine;
the various colours must be centered and the printing pressures adjusted to obtain exact position of all the colours;
the printing machine must then be started to achieve the required production.
The polymer printing plates can be prepared and mounted on the plate cylinders either manually or by means of appropriate devices known as xe2x80x9cplate assemblersxe2x80x9d, for example of the electronic or so-called xe2x80x9cpointxe2x80x9d type. However the operations involved in mounting the plates on their cylinders are extremely repetitive, whether they are mounted manually or by means of plate assemblers. In addition, each time a given client""s order is to be repeated, even if the printing has the same pitch as that underway, the plate cylinders have to be removed from the printing machine, the plates have to be mounted on the various cylinders and, when the production batch is complete, the plates have to be removed from the cylinders and be returned to store to remain there until the next order. As will be apparent, said operations are complicated, lengthy, repetitive and significantly affect the production cost.
Moreover the current market tendency (based on the xe2x80x9cjust in timexe2x80x9d concept) is to reduce the volume of an individual production batch and increase the order frequency, to consequently increase the number of times a certain production has to be repeated. At each production repeat, it is necessary in practice to replace the said double-adhesive tape by which the plates are applied to the cylinders, this tape being of considerable width and of consequent high cost per unit area.
In removing the polymer plates from the relative cylinders to store them until their next use, it not infrequently happens that the plates stretch and deform or even tear, with consequent involvement of cost for their replacement.
Furthermore, as is well known to the expert of the art, each time the plates are applied to the cylinders there is a very high risk of error consequent on forgetfulness (one or more plates are not mounted) or on inaccurate measurement of the printing pitch.
When the production batch is complete the cylinders have to be changed on the printing machine, with consequent machine down-time required to;
a) halt the machine;
b) withdraw the plate cylinder from the relative impression cylinder, this operation being required for each colour;
c) remove the caps from the pivots of the relative plate cylinder;
d) manually or automatically remove each plate cylinder from the printing machine;
e) arrange the plate cylinders on appropriate supports in a parking region provided for this purpose;
f) remove the plates applied to the cylinders and mount on the relative cylinders the plates of the new batch to be worked;
g) manually or automatically pick up the cylinders to which the new plates have been applied and transport them to the printing machine;
h) mount the plate cylinders on the printing machine;
i) centre the various colours and set the pressures;
j) restart the machine.
From the aforegoing it is apparent that the aforedescribed operations are substantially repetitive. It should however be noted that some of these operations are also dangerous for the operator (in particular the removal, transport and remounting of plate cylinders).
In any event the relative machine stoppage significantly affects production cost, even if this cost cannot be easily quantified precisely. However, to give an idea, reference will be made to a batch of about 2000 kg of polyethylene film of width 100 centimetres and thickness 50 microns printed with six colours. It is printed at a rate of about 120 metres per minute, with a resultant production time normally of about 6 hours, also taking account of possible machine down-times. The time required for changing the work, on the assumption that the new work also requires six colours, is 70-80 minutes, representing a certainly not negligible percentage of the overall batch production time.
An object of this invention is to provide a method for mounting printing plates on the relative cylinders of flexographic printing machines which enables the plant down-time involved in mounting or changing the plates to be substantially reduced.
A further object is to provide a plate mounting method which substantially reduces operator danger.
A further object of the invention is to provide a method of the aforesaid type which drastically reduces the possibility of error in applying the plates to the cylinders.
Said objects are attained according to this invention by a plate mounting method comprising the following stages:
applying those plates relative to a certain colour to a sleeve formed from a flexible transparent polymer film which is substantially indeformable in the plane of the film, ie which cannot be elongated on any side, this sleeve constituting the development of all or part of the surface of the relative plate cylinder; and
applying the sleeve to the relative cylinder.